An intersection built on pylons in the 1970s sticks up more than 12 feet higher than the road, as crews work.

By Chris Hawley, USA TODAY

MEXICO CITY  Deep underground, tunneling crews are racing against time as they try to save the world's third-largest metropolis from catastrophe.

Above them, the Mexican capital is sinking into the earth at a record rate, tilting the city's sewage tunnels so they are actually running backward. Crews are rushing to build a 37-mile drainage tunnel to save the city from drowning.

"Imagine the Congress, the stock exchange, the country's biggest airport, everything underwater," says Ariel Flores, water reuse manager for the National Water Commission. "It would paralyze the economy of the entire country. It would be a total disaster."

Across this city of 18.7 million, workers have started a flurry of projects to shore up areas that are sinking by as much as 8 inches a year. They're renovating a key intersection, filling holes under a commuter train line, reinforcing churches in the historic center, rehabilitating another drainage tunnel and dredging above-ground sewage canals.

Flooding poses the most danger, and there are already signs of trouble. In February, the Remedios River, a sewage canal, backed up and broke through its dike, flooding 4,000 homes with raw waste. Officials evacuated swaths of eastern Mexico City, worried about an epidemic deadlier than the H1N1 flu that swept over the city last year.

After the flood, the city installed five more massive pumps to force water out of the sinking metropolis.

"The goal: to reduce the risk of flooding as much as we can," Mayor Marcelo Ebrard says. "With luck, we'll be in time."

Mexico City's sinking problem dates back centuries. The Aztecs built their capital, known as Tenochtitlan, on a flat island in the middle of a lake. The city flooded frequently.

After the Spanish defeated the Aztecs in 1521, Spanish colonizers began draining the lake to control flooding. One flood, in 1629, left the city underwater for five years.

As the water disappeared, the city settled into the mud, forcing the government to build ever-deeper drainage tunnels to carry the water to lower ground.

In recent decades, the city's population soared, forcing authorities to pump more drinking water from underground aquifers and worsening the sinking. Mexico City and its suburbs make up the world's third-most-populous urban area after Tokyo and New Delhi, according to the United Nations.

Much of the wastewater no longer flows naturally out of the city. Pumps are used to get it over a rise called the Sierra de Guadalupe.

Landmarks anchored to bedrock are thrust skyward as the rest of the city drops. Workers have had to add 14 steps to the base of the Independence Angel monument since it was built in 1910, and a water pipe installed at ground level in 1934 now juts 27 feet in the air beside the Monument to the Revolution.

The Insurgentes Traffic Circle, a main intersection built on underground piles in 1970, is now 12 feet higher than the streets feeding into it.

On a recent afternoon, workers were tearing up asphalt to make way for a new ramp so drivers could make the grade without gunning their engines.

"We can't stop the sinking, so we just have to adapt the streets to it," engineer Carlos Pixor says.

In the city's colonial center, the federal government is injecting columns of wet concrete 115 feet beneath the Holy Trinity Church in an attempt to shore it up. The 17th-century church is listing to the side and sinking faster than the surrounding streets. Worshipers descend a 7-foot stairway to get in the door.

The biggest project is the Eastern Drainage Tunnel, a 23-foot-wide, $1.1 billion pipeline that will run north for 37 miles from Mexico City to lower ground in the Mezquital Valley.

At its deepest point, the tunnel will be 495 feet underground. Excavation began in June 2008 and will finish in 2012, the National Water Commission says.

It will carry away wastewater and reduce the risk of flooding, but it won't stop the sinking, says Ramón Domínguez Mora, a hydraulic engineer at the National Autonomous University of Mexico. To stop the ancient lake bed from contracting further, the city either has to stop pumping from its aquifer or inject water back into the ground, he says. But first the water must be cleaned, and the city does not have enough water treatment plants. Only 6% of Mexico City's sewage and rain runoff is treated; the rest is used to irrigate fields.

Even if the water could be returned to the aquifer, it's unlikely the ground could be "re-inflated," Flores says. Once the soil of the lake bed is compacted, it will not return to its original volume, he says.

"You can't raise the city again," Domínguez says. "The only hope is to stop it from sinking further."

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